The invention relates to a method and apparatus for increasing the density of a channel bank in a T-1 or other digital carrier system.
In a T-1 digital carrier system, a voice signal is sampled. The resulting pulse amplitude modulated (PAM) signal is converted to an 8-bit pulse code modulated (PCM) digital signal which is interleaved with 23 other channels for transmission over a T-1 line at a bit stream rate of 1.544 megabits per second (Mbps). T-1 signals are processed in a channel bank such as the channel bank 20 depicted in FIG. 1. In a conventional digital channel bank, 24 channels are collectively referred to as a digroup. Channel banks typically comprise two digroups A and B to create a 48-channel framework for transmitting and receiving on two duplex T-1 carriers.
As shown in FIG. 1, the channel bank comprises a chassis 24 having physical card slots 26 into which at least 48 channel unit (CU) cards 28 can be inserted, as well as a number of common equipment cards 30 described below. A chassis 24 typically has four rows of card slots with twelve physical card slot locations per row. Two rows of CU cards can constitute a digroup which uses the 24 channels of a T-1 connected to the channel bank. The CU cards 28 and the common equipment cards 30 communicate with each other via a backplane which comprises a pulse amplitude modulated (PAM) bus for analog signals and a pulse code modulated (PCM) bus for digital signals. The backplane 32 is illustrated in FIG. 2 with respect to two cards 28 from two shelves inserted in the chassis 24. The cards have edge connectors 34 with pins 36 that are electrically connected to terminals 38 on the backplane when the cards are inserted into the physical card slots 26 of the channel bank chassis 24.
FIG. 3 depicts CUs and common equipment associated with a conventional D4 channel bank 40 for illustrative purposes. Twenty-four channel units 42A1 through 42A24 and twenty-four channel units 42B1 through 42B24 constitute the voice/data circuits of digroups A and B. Each channel unit in the digroup A is connected to a transmit unit A (TU A) 44A and to a receiver unit A (RU A) 46A. Similarly, each channel in the digroup B is connected to a transmit unit B (TU B) 44B and to a receiver unit B (RU B) 46B. Each digroup A and B has an alarm control unit (ACU) 48A and 48B, respectively. The common equipment also includes a line interface unit (LIU) 50, a trunk processing unit (TPU) 52, a transmit pre-equalizer 54, an office interface unit 56, a DC-to-DC converter 58 and a power distribution unit (PDU) 60.
The central office floor space of regional Bell Operating Companies (RBOCs) is increasingly in short supply. The ongoing addition of bulky D4 bays to meet increasing consumer demands for telecommunications service, as well as the increasing number of non-RBOC carriers that are co-located within the same office, has exhausted available room or floor space in some central offices. Accordingly, a need exists for a channel bank with increased density, that is, a channel bank which can process more channels (e.g., more than 48 channels) than conventional channel banks in relation to the amount of room a channel bank occupies in a central office or other RBOC facility. A number of channel banks and channel unit cards have been proposed which allow a reduction in the physical size of a channel bank. These channel banks and cards, however, are disadvantageous because they are not easily retrofitted into existing D4 bays, and do not allow for the use of existing channel unit cards and common equipment. Thus, a need exists for a channel bank architecture which increases the number of T-1 channels handled thereby, and which also allows for plug-and-play upgrades to existing D4 bays.
In accordance with the present invention, a double density channel bank architecture is provided to overcome the problem of diminishing central office space to accommodate additional channel banks.
In accordance with one aspect of the present invention, the double density channel bank architecture is compatible with an existing D4 backplane, as well as existing D4 channel units and a number of common equipment components. Double density channels units and common unit components are provided which can be inserted into an existing channel bank for upgrading to accommodate additional channels per slots.
In accordance with another aspect of the present invention, a channel unit is configured to transmit or receive PCM data on the PAM bus of a channel bank, while simultaneously transmitting or receiving PCM data on the PCM bus. One double density channel unit can therefore provide data for two transmit and two receive T-1 timeslots to increase the capacity of a D4 channel bank from 48 channels or two duplex T-1 carriers to 96 channels or four duplex T-1 carriers.
In accordance with yet another aspect of the present invention, the channel bank architecture provides timing of the PCM data on the PAM bus to avoid bus contention when an adjacent conventional channel unit provides an analog sample on the PAM bus.
A method of increasing the number of digital carrier channels processed in a channel bank is provided comprising the steps of: (1) formatting a first signal for digital transmission on a first carrier channel if the first signal is not digital; (2) formatting a second signal for digital transmission on a second carrier channel if the second signal is not digital; (3) providing a portion of the second signal on a pulse code modulated bus of the channel bank, and (4) providing a portion of the first signal on a pulse amplitude modulated bus of the channel bank, the pulse amplitude modulated bus being operable to transport analog signals from analog cards in the channel bank, the portion of the first signal being provided on the pulse amplitude modulated bus using selected timing to avoid bus contention with the analog signals.
In a channel bank having a chassis comprising card slots for receiving channel units and common equipment components and operable to provide signals to duplex digital carriers, the common equipment comprising receive units, transmit units, a line interface unit and a trunk processing unit, the channel bank having a backplane comprising a pulse code modulated bus and a pulse amplitude modulated bus, the channel units providing a single subscriber circuit for transmitting and receiving signals via a single duplex channel on one of the duplex digital carriers, an improved channel bank architecture provides increased density of subscriber circuits in the channel bank without changing the card slots and the chassis. The improved channel bank architecture comprises an increased density channel unit configured for insertion in one of the card slots, the increased density channel unit being operable to provide a portion of a first second signal on the pulse code modulated bus of the channel bank and a portion of a second on the pulse amplitude modulated bus substantially simultaneously to support two of the subscriber circuits in the corresponding one of card slots.